1. Field of the Invention:
The present invention relates to the isolation and purification of isocyanate condensates containing free NCO groups, such condensates having been prepared from aliphatic, cycloaliphatic or arylaliphatic di- or polyisocyanates and the isocyanate groups of which are not directly bonded to an aromatic ring.
2. Description of the Prior Art:
It is well known to this art that isocyanate condensates are particularly useful for producing foams, elastomers, adhesives, paints and varnishes having desirable final properties.
These condensates containing free NCO groups are prepared by reacting at least one compound containing at least two functional groups which are reactive with isocyanates with a molar excess of di- or polyisocyanate, optionally in a solvent which is inert with respect to NCO groups. The compound containing functional groups which are reactive with NCO groups may thus contain --OH, --SH and --COOH groups as reactive sites, it being possible, of course, for the functional groups in such a compound to be identical or different. Exemplary of such compounds are diols and polyols. The compound bearing the groups which are reactive with NCO groups may be an aliphatic, cyclic, cycloaliphatic or aromatic diradical; it may itself also be a condensate of simple molecules or a diradical comprising heteroatoms in the molecule chain, if appropriate. The condensates containing free NCO groups may thus be prepared from the following starting materials:
(a) polyesters produced by esterification of one or more di- or polyols with one or more di- or polyacids, or by reacting a cyclic lactone with a di- or polyfunctional molecule containing --OH, --NH.sub.2 or --NHR groups, for example;
(b) polyethers produced by condensation of cyclic oxides (ethylene, propylene, butylene or tetramethylene oxide) with a di- or polyfunctional molecule containing --OH, --NH.sub.2 or NH groups; and
(c) mixed condensates containing polyether and polyester blocks.
These starting materials are reacted with an excess of di- or polyisocyanate, optionally in a solvent which is I5 unreactive with respect to isocyanate groups. Exemplary of such di- or polyisocyanates, representative are:
- 1,3-diisocyanatopropane; PA0 - 1,4-diisocyanatobutane; PA0 - 1,5-diisocyanatopentane; PA0 - 1,6-diisocyanatohexane; PA0 - 1,4-diisocyanato-2-ethylbutane; PA0 - 1,5-diisocyanato-2-methylpentane; PA0 - 1,6-diisocyanato-2,2,4-trimethylhexane; PA0 - 1,6-diisocyanato-2,4,4-trimethylhexane; PA0 - 1,2-diisocyanatocyclohexane; PA0 - 1,4-diisocyanatocyclohexane; PA0 - 1,2-bis(isocyanatomethyl)cyclobutane; PA0 - bis(4-isocyanatocyclohexyl)methane; PA0 - 3,3,5-trimethyl-5-isocyanatomethyl-1-isocyanatocyclohexane; PA0 - 1,4-bis(isocyanatomethyl)benzene; and PA0 - 1,2-bis(isocyanatomethyl)benzene.
These diisocyanates may be used separately or as mixtures of any number thereof.
Thus, for example, 1,6-diisocyanatohexane (HDI), which is one of the preferred diisocyanates, may be used alone, or mixed with 1,5-diisocyanato-2-methylpentane and/or 1,4-diisocyanato-2-ethylbutane; mixtures of these latter two diisocyanates may also be used.
The reaction of excess polyisocyanate with the antagonist compound is carried out according to known procedures, namely, by heating the mixture of both materials, optionally in the presence of a catalyst and/or a solvent.
The common feature of these condensates containing free NCO groups is that, when the reaction is complete, the reaction mixture includes a more or less considerable amount of the diisocyanate employed in excess. This diisocyanate must often be removed from the polycondensate produced, especially because of the toxicity due to the volatility of such diisocyanate. Indeed, when such condensates produced from a diisocyanate such as 1,6-diisocyanatohexane (HDI) are used, the presence of a more or less considerable amount of HDI can present health and safely hazards. These hazards are considerably more pronounced when the condensate is used as a thin layer, as is the case with paints and varnishes, for example, because the emission of toxic HDI vapor can become considerable and exceed the legally permitted limits in the surrounding atmosphere.
Such condensates, especially those produced by reacting polyesters or polyethers with an excess of diisocyanate (these condensates being designated "prepolymers" in most cases), for the manufacture of elastomeric materials, cellular or otherwise. In this case, the prepolymer is reacted with a stoichiometric amount of a reactant which is at least difunctional and which contains functional groups reactive with NCO groups, as indicated above. The final material will contain a more or less appreciable amount of the product of condensation of the excess diisocyanate with the difunctional reactant, the presence of which may impair the properties of the final elastomeric material. In this case, removal of the free diisocyanate would be the only way to produce a prepolymer having a low degree of condensation and a low free diisocyanate content at the same time.
The removal of the excess diisocyanate may be carried out by known means, such as evaporation of extraction with a solvent for the diisocyanate, but a nonsolvent for the condensate. However, evaporation requires the use of high temperatures, which may impair the quality of the condensate. This disadvantage is reflected in secondary reactions when heat is applied, resulting in a product of high viscosity which can form a deposit in the evaporator. Moreover, good exhaustion of the free diisocyanate requires the use of moving thin-film evaporators. These are costly devices and, in this case, mandate frequent shutdowns and cleaning operations, because, in most part, the molten condensate gradually forms a resin in the apparatus. Furthermore, extraction with a solvent for the diisocyanate which is a nonsolvent for the condensate (such as hexane, octane, etc.) is lengthy and cumbersome. In fact, as soon as the nonsolvent is added, the condensate tends to precipitate in the form of a sticky mass from which the free diisocyanate monomer is difficult to extract completely.